Citrate plays a pivotal role not only in the generation of metabolic energy but also in the synthesis of fatty acids, isoprenoids, and cholesterol in mammalian cells. Plasma levels of citrate are the highest (ϳ135 M) among the intermediates of the tricarboxylic acid cycle. Here we report on the cloning and functional characterization of a plasma membrane transporter (NaCT for Na ؉ -coupled citrate transporter) from rat brain that mediates uphill cellular uptake of citrate coupled to an electrochemical Na ؉ gradient. NaCT consists of 572 amino acids and exhibits structural similarity to the members of the Na ؉ -dicarboxylate cotransporter/Na ؉ -sulfate cotransporter (NaDC/NaSi) gene family including the recently identified Drosophila Indy. In rat, the expression of NaCT is restricted to liver, testis, and brain. When expressed heterologously in mammalian cells, rat NaCT mediates the transport of citrate with high affinity (Michaelis-Menten constant, ϳ20 M) and with a Na ؉ :citrate stoichiometry of 4:1. The transporter does interact with other dicarboxylates and tricarboxylates but with considerably lower affinity. In mouse brain, the expression of NaCT mRNA is evident in the cerebral cortex, cerebellum, hippocampus, and olfactory bulb. NaCT represents the first transporter to be identified in mammalian cells that shows preference for citrate over dicarboxylates. This transporter is likely to play an important role in the cellular utilization of citrate in blood for the synthesis of fatty acids and cholesterol (liver) and for the generation of energy (liver and brain). NaCT thus constitutes a potential therapeutic target for the control of body weight, cholesterol levels, and energy homeostasis.

NaCT (sodium-coupled citrate transporter) is an Na(+)-coupled citrate transporter identified recently in mammals that mediates the cellular uptake of citrate. It is expressed predominantly in the liver. NaCT is structurally and functionally related to the product of the Indy (I'm not dead yet) gene in Drosophila, the dysfunction of which leads to lifespan extension. Here, we show that NaCT mediates the utilization of extracellular citrate for fat synthesis in human liver cells, and that the process is stimulated by lithium. The transport function of NaCT is enhanced by lithium at concentrations found in humans treated with lithium for bipolar disorders. Valproate and carbamazepine, two other drugs that are used for the treatment of bipolar disorder, do not affect the function of NaCT. The stimulatory effect of Li+ is specific for human NaCT, since NaCTs from other animal species are either inhibited or unaffected by Li+. The data also suggest that two of the four Na(+)-binding sites in human NaCT may become occupied by Li+ to produce the stimulatory effect. The stimulation of NaCT in humans by lithium at therapeutically relevant concentrations has potential clinical implications. We also show here that a single base mutation in codon-500 (TTT-->CTT) in the human NaCT gene, leading to the replacement of phenylalanine with leucine, stimulates the transport function and abolishes the stimulatory effect of lithium. This raises the possibility that genetic mutations in humans may lead to alterations in the constitutive activity of the transporter, with associated clinical consequences.

Members of the R7 family of the regulators of G-protein signaling (R7 RGS)

We performed genome-wide chemical mutagenesis of C57BL/6J mice using N-ethyl-Nnitrosourea (ENU). Electroretinographic screening of the third generation offspring revealed two G3 individuals from one G1 family with a normal a-wave but lacking the b-wave that we named nob4.Th e mutation was transmitted with a recessive mode of inheritance and mapped to chromosome 11 in a region containing the Grm6 gene, which encodes a metabotropic glutamate receptor protein, mGluR6. Sequencing confirmed a single nucleotide substitution from T to C in the Grm6 gene. The mutation is predicted to result in substitution of Pro for Ser at position 165 within the extracellular, ligand-binding domain and oocytes expressing the homologous mutation in mGluR6 did not display robust glutamate-induced currents. Retinal mRNA levels for Grm6 were not significantly reduced, but no immunoreactivity for mGluR6 protein was found. Histological and fundus evaluations of nob 4 showed normal retinal morphology. In contrast, the mutation has severe consequences for visual function. In nob4 mice, fewer retinal ganglion cells (RGCs) responded to the onset (ON) of a bright full field stimulus. When ON responses could be evoked, their onset was significantly delayed. Visual acuity and contrast sensitivity, measured with optomotor responses, were reduced under both photopic and scotopic conditions. This mutant will be useful because its phenotype is similar to that of human patients with congenital stationary night blindness and will provide a tool for understanding both retinal circuitry and the role of ganglion cell encoding of visual information.

An electroretinogram (ERG) screen identified a mouse with a normal a-wave but lacking a b-wave, and as such it was designated no b-wave3 (nob3). The nob3 phenotype mapped to chromosome 11 in a region containing the metabotropic glutamate receptor 6 gene (Grm6 RGCs. Grm6nob4/nob3 RGC responses verified the conclusion that the two mutants are allelic. We propose that Grm6 nob3 is a new model of human autosomal recessive congenital stationary night blindness. However, an allelic difference between Grm6 nob3 and Grm6 nob4 creates a disparity in inner retinal processing. Because the localization of GRM6 is limited to bipolar cells in the On pathway, the observed difference between RGCs in these mutants is likely to arise from differences in their inputs.